1. Field of the Invention
The present invention is a mask and a process for fabricating that mask. More specifically, it is a mask that combines the properties of an attenuated phase shifting mask (Att PSM) and an alternating phase shifting mask (Alt PSM). The method of fabrication achieves simplicity and cost-effectiveness by combining different intensities of electron-beam developing in a single resist medium.
2. Description of the Related Art
As structural dimensions in microelectronic circuitry become increasingly small, lithographic processes that image the circuit design on semiconductor substrates are called upon to resolve details of 0.35 microns and less. The use of optical lithography in this resolution regime is severely limited by diffraction effects caused by the small openings in the masks which are used to transfer circuit patterns by optical imaging. Diffraction of the transmitted light blurs the edges of small isolated structures and severely limits the ability to resolve spacings between densely packed and closely neighboring structures. Two new technologies have emerged that significantly improve the resolution of the masks used in optical lithography: alternating phase shifting (Alt PSM) and attenuated phase shifting (Att PSM). The concept of phase shifting is central to both technologies. Phase shifting is the translation of an electromagnetic sinusoid by a given angle (the phase angle) relative to a reference wave. When a shifted and un-shifted wave combine on a common image plane, the resulting superposition can produce constructive or destructive interference in the fields depending on the size of the phase shift and concurrent increases or decreases in the field intensities. If the un-shifted wave produces a diffraction pattern by passing through a small aperture in a mask, the unwanted diffraction lobes on either side of the central maximum can be significantly reduced by also passing the wave through an adjacent aperture that is filled with a transparent medium fabricated so as to shift the phase by 180 degrees (.pi. radians). A series of adjacent apertures, with alternate apertures filled with a phase shifting medium, forms what is called an alternating phase shifting mask (Alt PSM). Such masks are discussed in C. Y. Chang and S. M. Sze, "ULSI Technology," McGraw-Hill Co., N.Y. 1996, pp 284-288. The construction of a mask by combining alternating phase shifted and non-phase shifted structures for bi-directional circuit patterns is discussed by Lin in U.S. Pat. No. 5,472,814, 1995. The application of combined alternating phase shift masks to the reproduction of complex circuit patterns is also discussed by Lin et al in U.S. Pat. No. 5,523,186, 1996.
A second technology utilizing the properties of combined shifted and un-shifted electromagnetic radiation, is the attenuated phase shift mask (Att PSM). This design surrounds an isolated opening in a mask with a material such as molybdinum silicide oxynitride (MoSiON) that both phase shifts and attenuates the transmitted radiation on either side of the opening, only where the unwanted diffraction lobes of the non-phase shifted radiation appear. This combination does not affect the central maximum that delineates the opening, but it significantly reduces the intensity of the diffraction lobes on either side. This has the effect of sharpening the image of the opening. The attenuated phase shift mask is discussed by Tzu et al, U.S. Pat. No. 5,783,337, 1998.
The Alt PSM technology is effective for improving the resolution of repetitive, closely packed structures, but it provides no benefits when applied to isolated structures. Conversely, the Att PSM technology improves the resolution of small, isolated structures, but offers no advantages where closely packed structures are involved.
A way of achieving the improved resolution benefits of phase shifting in its various forms where the circuit design to be imaged has both densely packed and isolated structures, is to combine the alternating and attenuated phase shifting technologies on the same mask. This is the approach taken by Lin et al in U.S. Pat. No. 5,565,286, 1996. Although this combination of technologies is an excellent way to produce high resolution images of complex circuit designs having a variety of structural forms, the fabrication of such a mask involves a large number of separate processing steps as well as a multiplicity of materials, both leading to increased production time and cost. The method of Lin et al (U.S. Pat. No. 5,565,286) cited above, for example, employs separate and multiple layers of different material to act as phase shifters and attenuators of different transmissivities, so that the completed fabrication has as many as five layers of active material. A more cost-effective fabrication process for a combined Alt PSM and Att PSM on the same mask would be of great benefit and is the subject of the invention described herein.